(1) Field of the Invention
The present invention relates to protective fabric, and more particularly to a method for producing a crimp-imbalanced fiber for use as fabric components of soft protective armor.
(2) Description of the Prior Art
Crimped fabrics, such as a plain-woven construction example shown in FIG. 1, uniquely develop architectural changes on the meso-scale (yarn-to-yarn level) through crimp interchange as functions of biaxial tensions. Crimp interchange enables the tensile forces among yarn families to vary with applied multi-axial loading.
Crimp is typically defined as the waviness of a fiber in fabric form. Crimp interchange is the transfer of crimp content from one yarn direction to the other(s) as a consequence of fabric loading. Crimp interchange results from the relative motions of slip and rotation between yarn families at the yarn crossover points in response to applied loads. Crimp interchange is dependent upon the ratio of initial crimp content among yarn families and the ratio of stress between yarn families rather than the levels of stress. Crimp interchange, which is a coupling mechanism analogous to Poisson's effect in traditional materials, produces substantial nonlinearities in the constitutive behavior of woven fabrics.
FIG. 2 and FIG. 3 identify crimp-related parameters in geometric models for plain-woven fabrics constructed of yarns of circular cross sections. The two types of crimping shown in the figures describe two possible cases for a fabric. The parameters (recognizable to those ordinarily skilled in the art) for interpreting the use of the figures are: “d” is the yarn diameter (the same for the weft and warp yarns); “D” is the fabric thickness measured at cross-over points (overlap regions where the warp yarns cross the weft yarns); “p” is the distance between centers of adjacent yarns; “h” is the distance between centerlines of adjacent weft yarns (and h/2 is just one-half of h, also note that when h=0, there is no crimp in the weft yarns); “alpha” is the crimp angle of the warp yarns; and “L/2” is one quarter of the warp yarn's wave length shape (note that 4×L/2 equals one complete wave length of the warp yarn shape).
In the uni-directional crimp case depicted in FIG. 2, the yarns 2, 4 and 6 are not crimped. The yarns 2, 4 and 6 lie straight in the same horizontal plane and have zero waviness. Yarn 10 is crimped (having waviness) to allow placement amongst the other yarns 2, 4 and 6. Therefore, this type of fabric construction is said to be uni-directionally crimped—only one yarn family 10 has crimp content (ie: waviness). The bi-directional crimping of FIG. 3 depicts that both yarns families; that is yarns 2, 4, 6 and 10 have waviness (note that the yarns 2, 4 and 6 do not lie within the same horizontal plane—see reference line 12).
The parameters of FIG. 3 are applicable for defining the geometric dependencies of crimp in fabrics constructed with tows (non-twisted yarns) of a nearly rectangular cross-section. Many ballistic fabrics employ non-circular cross section yarns such as rectangular, lenticular, elliptical, etc. Each type of yarn cross section provides slightly different sliding, interlocking, shearing and compaction compression at the crossover points) characteristics at the points when the fabric is subject to extensional and shearing forces.
Crimp content is obtained by measuring the length of a yarn in a fabric state, Lfabric, and the length of the yarn after extraction from the fabric, Lyarn, and straightened out according to Equation (1).
                    C        =                                            L              yarn                        -                          L              fabric                                            L            fabric                                              (        1        )            
There exists a limiting phenomenon to crimp interchange. As the biaxial tensile loads continually increase, a configuration results in which yarn kinematics (i.e.; slip at the crossover points) cease and the interstices (spaces) between converge to minimum values. This configuration is referred to as the extensional jamming point. The jamming point can prevent a family of yarns from straightening thus limiting stresses in those yarns and in extreme cases can avert tensile failures. With the absence of failures in those yarns during a ballistic impact event, these yarns remain in position to provide a blunting mechanism that distributes the impact forces over a progressively larger number of yarns in subsequent fabric layers.
Research investigating ballistic impact mechanics of crimped fabrics have recognized the role of crimp interchange. Crimp interchange is often explored together with inter-yarn friction mechanisms because both involve sliding interfaces among yarn surfaces at the crossover points.
Research in woven ballistic fabric armor has produced findings that: (1) generally purport ranges of desirable friction coefficients for optimal ballistic protection performance measured in terms of a V50 designation; (2) identify limiting bounds of these coefficients for use in numerical and analytical models and (3) establish the need for sizing methods to affect fiber roughness. Ballistic protection limits are designated by V50, which is the velocity at which an armor panel of a given areal density has a 50% probability of stopping the projectile at zero degree obliquity.
Crimp effects in structural fabrics have also been researched. In the area of pneumatic structures, air beams were researched to establish the combined biaxial and shear behavior of plain-woven fabrics. Both meso-scale unit cell and fabric strip models were validated. The results indicated that crimp interchange, decrimping and shearing (also referred to as trellising—FIG. 4, FIG. 5 and FIG. 6) play major roles in the mechanical response of crimped fabrics subjected to applied structural forces. FIG. 4 depicts an unloaded state of woven fabric; FIG. 5 depicts a shearing (trellising) state of woven fabric and FIG. 6 depicts a shear jamming stage of woven fabric.
Shear trellising and shear jamming are the terms given to the configuration of a fabric subjected to pure shear. Consider the lower ends of the vertical yarns 20 clamped and the right ends of the horizontal yarn 22 clamped. Now, consider a horizontal force applied to the upper end of the vertical yarns 20. This is the shearing mode of loading that will cause the yarn rotations (trellising) and eventual yarn jamming states.
The advantageous effects of functionally graded crimping by design on soft fabric armor have not been sufficiently explored as a mechanism for increasing the combined ballistic and penetration protection levels as well as flexibility.
In the prior art, U.S. Pat. Nos. 6,720,277; 6,693,052; 6,548,430; 5,976,996; 5,837,623; and 5,565,264 of Howland relate to fabric substrates of woven constructions having principally two yarns, namely warp and fill (also referred to as weft), aligned in an orthogonal layout in accordance with a plain-woven architecture. These cited references claim a variation of crimp contents between the warp and weft yarn directions within a single layer but do not achieve the improved performances to: reduce regions of oblique susceptibility through the use of bias yarns; employ bias yarns in conjunction with woven fabrics; enable functionally graded protections against ballistic and/or stab penetration threats using crimp gradients in the through-thickness direction of multi-layered fabric system and reduce blunt trauma.
Furthermore, the cited references of Howland describe plain-woven fabrics possessing cover factors (CF) up to one hundred percent for warp fibers at the weft center and in excess of seventy-five percent for the weft. It has been defined in the art that a cover factor on the geometrical sense as the fraction of orthogonally projected fabric area that is occupied by yarns. As the cover factor increases so does penetration protection because the interstices between yarns decrease in size, which increases the resistance of the yarns to be pushed aside by sharp pointed penetrators.
Technology advances in soft fabric armor designs have focused on two principal construction methods (layered woven armor systems and uni-directional, cross-ply, layered armor systems). FIG. 7 depicts uni-directional layers arranged in multiple 0/90 degree stacks. During a ballistic impact, the uni-directional yarns dissipate the kinetic energy rapidly due to the absence of yarn crossover points. The crossover points in woven fabric armor reflect the shock waves rather than absorb the shock waves to a reduction of absorbed energies.
A disadvantage of uni-directional constructed fabric armor is the trade in comfort and flexibility for the incremental increase in ballistic protection. While this does not present a usability issue for vehicle and structural armor, it can be an issue for flexible body armor. This is because uni-directional fabric armors are not interlaced; that is, no yarn crossover points exist to enable the relative motions among yarn families that produce flexibility and conformity.
A need therefore exists for technological advances in fiber architecture and therefore an advance in soft fabric armor.